How have the flood maps been prepared?

Flood maps are based on historical flood records as well as computer models. Because some large and rare floods have often not been experienced since European settlement, computer models are used to simulate the depths and velocities of major floods. These computer models are normally established and operated by flooding experts employed by local and state government authorities. Because of the critical importance of the flood level estimates produced by the models, such modelling is subjected to very close scrutiny before flood information is formally adopted by government.

Generally, flood mapping, determined by flood modelling techniques, has been carried out for many cities, and towns. Here flood understanding is crucial for land use and flood response planning to determine possible flood mitigation controls. Although some rural areas throughout Victoria have detailed flood modelling, most flood mapping has, to date, relied on past historical flood information, including aerial photography, satellite imagery, newspaper accounts, recorded peak flood levels, ground contour information etc.

Why do flood levels change over time?

It is inevitable that floods of various magnitudes will continue to occur in the future. As the size of a flood increases, the smaller the chance that it will occur. Because some of these rare floods have never been experienced since European settlement, the height of future floodwaters is normally predicted using computer models. These computer models simulate flood levels and velocities for a range of flood sizes and flood probabilities. Given the importance of estimating flood levels accurately, experts are used to establish and operate the computer models. From time to time the computer models are revised and predicted flood levels can change. The resultant change in flood levels however is normally very small. The reasons why the computer models are revised can include:

new rainfall or ground topography information becomes available;

new floods occur which provide additional data from which to fine-tune the models. New flow data of a large flood can influence the flood probabilities that sometime alter the design 100-year ARI flood levels;

better computer models become available as the science of flood modelling improves and computer capabilities increase; or

flood mitigation works may have been carried out, or development within the catchment may have occurred, that was not previously simulated in the models.

What is the probable maximum flood (PMF)?

The PMF is the largest flood that could possibly occur. It is a very rare and improbable flood. Despite this, a number of historical floods in Australia have approached the magnitude of a PMF. Every property potentially inundated by a PMF will have some flood risk, even if it is very small.

What is a 100 year flood?

A 100 year flood is the flood that will occur or be exceeded on average once every 100 years. It has a probability of 1% of occurring in any given year. If your area has had a 100 year flood, it is a fallacy to think you will need to wait another 99 years before the next 100 year flood arrives. Floods do not happen like that. Two such floods can occur one after the other as happened in Kempsey NSW in 1949 and 1950.

In 2007 alone rainfall exceeding the 1 in 100 event has occurred in Flinders Ranges SA, Campbelltown NSW, Newcastle NSW, Macalister River Vic, Sunshine Coast QLD. On average, if you live to be 70 years old, you have a 50/50 chance of experiencing a 100 year flood in any one location.